1. Field of the Invention
The invention relates to a drafting apparatus, and more particularly, to a drafting apparatus disposed in a furnace to isolate and reduce the burning region.
2. Description of the Related Art
Thermal process is a very important technique in semiconductor fabrication such as annealing, thermal oxidation, and thermal diffusion. Currently, thermal fabrication equipment is categorized into a horizontal type and a vertical type. The operating theory of both types is similar. The follow description is thus taking the horizontal type as a example to introduce.
In FIG. 1A, a top view of a horizontal type thermal furnace is shown. The thermal furnace comprises an annealed quartz tube 110, a 3-zone heating element 120 to heat the quartz tube 110 to a certain temperature, and an air injector 130 to inject air or gas into the quartz tube 110 to be heated. The 3-zone heating element 120 is a heater to heat a front end, a middle part, and a rear end of the quartz tube 110, respectively, so as to adjust the heat distribution of the quartz tube 110. A number of wafers 102, for example, about 100 to 150 wafers, is disposed on an annealed quartz wafer boat 106 to be placed into the quartz tube 110. By the heat provided by the 3-zone heating element, the quartz tub 110 is heated to a required temperature.
In a wet oxidation process, hydrogen (H.sub.2) and oxygen (O.sub.2) flow through two different intakes to the injector 130. Hydrogen and oxygen mix with each other and are injected into the quartz tube 110 by the injector 130. While igniting the mixture of hydrogen and oxygen, highly purified water steam is obtained to perform a wet oxidation process on the wafers 102. The amount of hydrogen is adjustable to avoid that while the oxygen is consumed, the remaining hydrogen causes an explosion in the furnace.
FIG. 1B schematically illustrates the injector 130. Using an axis 100 as a reference, the injector 130 has a tilt angle .theta..sub.1 for gas injection. Typically, the tilt angle .theta..sub.1 is no more than 10.degree.. While hydrogen is ignited, a high temperature oxy-hydrogen flame 132 and water steam are produced as shown in the figure.
As shown in FIG. 1C, to avoid the wafers carried aside on the wafer boat having a heat energy different from that of the wafers carried amid on the wafer boat 106, dummy wafers 104 are disposed aside on the wafer boat 106 instead of wafer products, while wafers 102 are disposed enclosed by the dummy wafers 104. Therefore, the heat energy is evenly and uniformly distributed to each wafer on the wafer boat 102. The number of the dummy wafers 104 is typically between 6 to 10 as specifically required.
In the above mentioned furnace, the effective region of the oxy-hydrogen flame 132 is long. In order to provide a substantially equal temperature to each, the wafer boat has to be disposed in a long distance away from the oxy-hydrogen flame 132, so that the temperatures of front end and the rear end of the wafer boat are substantially the same. In this manner, a large space is required. The amount of wafers to be carried in the wafer boat is limited to seriously affect the throughput of products.
However, the internal furnace temperature is difficult to control at all points, due to the long, effective region of the oxy-hydrogen flame 132. The front end of the quartz tube 110 has a temperature very much different from the temperature of the rear end of the quartz tube 110. As a consequence, the uniformity between wafers is reduced and disadvantageous to the subsequent fabrication process.
In addition, the effective region of the oxy-hydrogen flame 132 is not isolated from the wafers. The high temperature water steam is reacted with the wafers without being drafted. Thus, the reaction regions above the wafers are not uniform to affect the quality of the thermal process.
To solve the problem, an external ignition apparatus is developed. While the external ignition apparatus ignites, a high temperature wafer steam is lead into the quartz tube 110. However, since the ignition apparatus is disposed externally, once the hydrogen explodes, the operators are directly in danger. In addition, while cleaning the furnace, the external ignition apparatus has to be detached first and installed after the cleaning process. The above described procedures are both time and labor consuming. Thus, it is not very applicable in terms of safety and efficiency.
In summary, the disadvantages of the conventional thermal process comprises:
1. The utility space is highly reduced to reduce the throughput of products, so that the fabrication cost is increased. PA0 2. The uniformity of the heat region above the wafers is compromised, due to the long expanse of the oxy-hydrogen flame. PA0 3. Alternatively, an external ignition apparatus is used to replace the oxy-hydrogen flaming system. The external ignition apparatus causes a great problem in safety for operators. Furthermore, the efficiency is poor. PA0 4. The water steam flows in random direction, so that the reaction regions above the wafers are non-uniform to cause a quality problem of thermal process.